Wireless communication systems are normally based on a transmission of information between a transmitter and a receiver, whereby the information can be modulated on a carrier signal. In radio communication systems, the signal transmitted by the transmitter is captured by one or more antennas at the receiver end, and converted into an electrical signal, which can then be processed so as to extract the relevant information from the signal.
Many wireless communication systems, such as GSM900, GSM1800, UMTS, etc., operate within narrow frequency bands, centred around a centre frequency (such as 900 MHz in the case of GSM900, 1800 MHz in the case of GSM1800, 2000 MHz for UMTS, etc.). Thus, in order to obtain an adequate gain within the relevant frequency band, antenna systems are used that are tuned to the respective frequency band and that include at least one antenna having a resonant frequency substantially corresponding to the centre frequency of said frequency band.
A problem frequently involved with antennas is their size. For example, a dipole antenna having a resonant frequency f should have a length of about λ/2, where λ is the wavelength corresponding to said resonant frequency (such a dipole is often referred to as half-wavelength dipole). A monopole antenna mounted over a ground-plane should have a length of about λ/4, where λ is the wavelength corresponding to said resonant frequency (such a monopole is often referred to as quarter-wavelength monopole). Even for communication systems based on high frequencies (such as GSM900, GSM1800 and UMTS2000, corresponding to wavelengths of approximately 33 cm, 16 cm and 13 cm, respectively), obtaining sufficiently small and still useful antennas has been considered to be a difficult task (normally, external and/or retractile antennas have been used, or the antennas have been helically arranged, so as to reduce the space they occupy in the handheld devices).
In communication systems using lower frequencies and, thus, longer wavelengths, such as FM (operating in the band of 88-108 MHz) and DVB-H (Digital Video Broadcast-Handheld) (operating in the bands of 470-702 MHz, or 470-770 MHz), the long wavelengths can imply that the typical monopole and dipole antennas can be inappropriate for handsets (such as handsets for mobile radio communication, that normally have quite reduced dimensions). For example, at a typical DVB-H centre frequency of 586 MHz, a typical quarter wavelength (λ/4) monopole antenna would have a length of approximately 12.8 cm. Such a long antenna would not be suitable for a pocket size mobile handset (today, consumers are used to pocket-sized handsets with internal antennas). The size of the external antenna could be reduced by implementing it as a helical wire, but the reduction of the size would imply a reduction of the bandwidth.
However, in many cases, a large bandwidth can be desired. For example, for DVB-H applications, a bandwidth encompassing the band of 470-702 or 470-770 MHz could be desired, together with a gain of not less than −10 dB to −7 dB over said band. Antenna elements are very selective in terms of gain and bandwidth. Although a larger bandwidth can be obtained using lumped components, these components do not provide for an increased gain.
That is, one of the challenges involved with including a TV service in a portable or handheld device relates to the need to cover the wide spectrum that is usually allocated for TV services. For instance, as mentioned above, the DVB-H service in Europe should cover a bandwidth including the 470-770 MHz band (UHF), which implies a relative bandwidth of approximately 50% with respect to the center frequency of said operating band. Other digital and analog TV services and standards, particularly those using the terrestrial broadcast network, would also encompass such a large 50% relative bandwidth at similar frequencies within the VHF-UHF bands.
There is a well known trade-off between antenna size and bandwidth coverage. The smaller the antenna, the smaller the bandwidth. Typical prior art internal antennas for handheld devices feature a 5-15% relative bandwidth at even shorter wavelengths, such as those of cellular, mobile and wireless services (800 MHz-2200 MHz). When an internal antenna is operated outside its typical relative 5-15% bandwidth, the gain, the efficiency and the matching characteristics (VSWR, return-loss) of the antenna become severely degraded, at times to unacceptable levels.
The specifications and characteristics of a digital TV antenna for a portable or handheld device can be very different from those of the internal antennas for conventional mobile services. While a conventional mobile service antenna would require a VSWR<3, a gain better than −2 dBi (dB(isotropic) is the forward gain of an antenna compared to an idealized isotropic antenna) within a 5-15% relative bandwidth (or bandwidths in case of multiband services), a digital TV antenna is usually specified to cover a 50% relative bandwidth with a gain between −10 dBi and 4 dBi or better and a return-loss of −2 dB or better.
Some prior art devices make use of an external antenna, often a mechanically retractable external antenna, to cover TV services. Nevertheless, the use of an external antenna on a small portable device is inconvenient in terms of size (the length of such an antenna is often 7 cm or more), ergonomy, aesthetics, mechanical robustness and durability. It is one of the purposes of the present invention to provide an arrangement for a handheld device including an internal antenna system which is able to provide for the reception of TV services.
An antenna can be characterized by its input impedance, Zin=Rin+jXin (that is, the impedance has a real component (the resistive component or resistance) and an imaginary component or reactance (that can be capacitive or inductive). At the resonant frequency (or, as a resonating element normally has more than one resonant mode, at one of the resonant frequencies, for example, at the lowest resonant frequency) the imaginary component equals 0, that is, Xin=0.
The gain of an antenna system, also referred to as antenna system's efficiency, depends on several features, including the radiating efficiency of the antenna element (which is frequency dependent) and the matching (which is also frequency dependent). “Matching” refers to the reduction of miss match losses that have to be subtracted from the radiating efficiency. Normally, for each antenna, a matching network is used that is adapted to the characteristics of the antenna, including its input impedance. However, as the input impedance is frequency dependent, it can be difficult to provide a matching network that provides an adequate gain all over a wide frequency band. That is, in wide-band communication systems, such as FM and DVB-H, it can be difficult to provide a suitable matching network.
It is important to stress that the reception of the TV signal is limited not only by the design of the antenna, but by the design of the whole device. Usually, the device will include one or more printed circuit boards (PCB) embedding one or more ground-planes. Such a ground-plane is also part of the antenna system and its size has an effect on the quality of reception. In general, the smaller the ground-plane, the smaller the covered bandwidth. It is one of the purposes of the present invention to provide an antenna system for a handheld device which includes a comparatively small ground-plane, as this makes it possible to further reduce the size of the handheld device.
Jari Holopainen, et al. “Antenna for Handheld DVB Terminal” 2006 IEEE International Workshop on Antenna Technology: Small Antennas, Novel Metamaterials, pp. 305-308, held at White Plains, N.Y., Mar. 6-8, 2006, discloses an example of an antenna allegedly useful for DVB terminals. Here, the EMC shielding and the printed circuit board (PCB) of a handheld terminal are considered to define a metal box, or chassis, which is utilised as part of the antenna. In accordance with the disclosure, surface currents are induced to the chassis capacitively at one end of the chassis, where the electric field has a maximum. The feed is stated to be a non-resonant and practically non-radiating compact coupling element. It is further stated that because of the non-resonant structure, the resonance needs to be achieved outside the antenna, for example, with a matching circuit.
The coupling element is a substantially L-shaped element which is arranged in correspondence with one of the shorter ends of a rectangular ground-plane constituted by the metal layer of a PCB. This shorter end has a width of 75 mm, and the L-shaped element has the same width. It appears that the non-resonant condition of said L-shaped element is due to its limited length with respect to the DVB-H wavelength. However, a 75 mm long conducting element in a coupled antenna structure such as the disclosed one may still provide for a suitable frequency response, when a suitable matching network is used, as suggested in the above-mentioned document.
However, for many handset applications, a width in the order of 75 mm may be inconvenient or unacceptable. This is, for example, the case in many mobile radio communication handsets, where the size is essential.
Thus, there is a need to find an appropriate antenna arrangement suitable for communication systems requiring a large bandwidth and that allows the antennas to be conveniently housed inside small size devices such as handsets, while at the same time providing for the necessary gain.